1. Field of the Invention
This invention relates to an imaging endoscope employing gradient index of refraction optical materials in its objective, and, in certain embodiments, in its transfer module. The endoscope of this invention can be manufactured at sufficiently low cost to be cost-effective for single-patient, disposable applications
2. Description of the Prior Art
Substantial benefits are available by performance of endoscopic surgical procedures rather than conventional "open" surgical procedures. More particularly, in an endoscopic surgical procedure the surgeon views the body joint, organ, or other structure to be examined through an elongated optical instrument referred to as an endoscope inserted through a relatively small incision, and performs whatever procedure is deemed necessary using a similarly elongated instrument (also typically referred to as an endoscope) inserted through the same or another small incision. By using these techniques, a majority of the trauma associated with traditional "open" surgery, involving much larger incisions and dissection of obscuring structures, is avoided. The result is substantially reduced health care costs and patient recovery time. For these reasons the use of endoscopic surgical techniques is increasing rapidly.
As used hereinafter, the term "endoscope" refers to an imaging endoscope, that is, an elongated optical instrument through which a surgeon views the surgical site. In general, an endoscope includes an objective at the distal tip of the endoscope for forming the image of the surgical site, an ocular which may include an eyepiece for direct viewing or may present the image to a video camera for displaying a video image of the site, and an elongated transfer module for transferring the image from the objective to the ocular. The objective may also include a prism if it is desired that the center of the field of view of the endoscope be at an oblique angle to the optical axis of the elongated probe.
Conventional endoscopes involve complex optical designs including a large number of glass elements, and are relatively difficult to fully sterilize. This is a particular problem given the present increased fear of infection. Further, many conventional endoscopes are relatively fragile and are expensive to repair. Accordingly, there is a substantial need in the art for a disposable endoscope satisfactory for forming an image of a surgical site of interest that is manufacturable at low cost so as to be cost-effective for single-patient, disposable applications.
The prior art has attempted to provide disposable endoscopes. In general, however, the cost of the conventional glass optical elements employed, typically having numerous individually polished spherical optical surfaces, has been so high as to render these instruments too expensive for single-patient, disposable use.
The prior art has suggested processes for forming aspheric optical surfaces on glass elements, in order to limit the number of individual glass elements required. To date no cost-effective process is available for doing so. To overcome the cost of glass optical elements, and also to overcome the limitation to spherical optical surfaces, the prior art has suggested employment of plastic optical elements that may readily be molded with aspheric surfaces where necessary. However, again the prior art has not to date provided an optically satisfactory endoscope manufacturable in a cost-effective fashion for single-patient, disposable use.
In further attempts to overcome these limitations, the art has also proposed manufacture of endoscopes using optical materials having a gradient in the index of refraction. Such "GRIN" materials are well characterized and are being employed in increasing numbers of optical devices.
The basic technique for forming GRIN materials is to diffuse atoms of elements altering the index of refraction of glass into the glass under appropriate conditions of temperature and pressure. With respect to generally cylindrical optical elements, the prior art suggests both "axial GRIN" materials wherein the index of refraction varies axially, that is, along the optical axis of an individual optical element, and also "radial GRIN" materials, that is, wherein the index of refraction is higher along the optical axis than at the radial periphery of the element, or vice versa. The variation in index of refraction for both axial and radial GRIN materials is symmetric about the optical axis. Radial GRIN optical elements in which the index of refraction is higher at the optical axis than at the radial periphery are referred to as having a positive gradient index of refraction, or "positive GRIN", while those having a higher index of refraction at the periphery than at the optical axis are termed "negative GRIN".
While as indicated axial and both positive and negative radial GRIN materials are suggested by the art for use in various optical instruments, in fact axial GRIN materials are not commonly employed in the design of optical instruments. Moreover, referring to radial GRIN elements, it is substantially more difficult to manufacture negative GRIN elements than positive GRIN elements. At the present time, only positive GRIN elements are manufactured in any significant quantity. Those of skill in the art will recognize accordingly that when a GRIN element is mentioned in the prior art without further description, it may be safely assumed that a positive radial GRIN element is intended.
With respect to specific discussion of the use of GRIN materials for endoscopes, U.S. Pat. No. 4,515,444 to Prescott et al teaches the employment of GRIN rod lenses with a spacer rod of a homogeneous (that is, non-GRIN) material between the objective and the transfer module of an endoscope. Prescott et al suggests that the GRIN rod lenses sold under the trade name "SELFOC" by Nippon Sheet Glass Co. of Japan can be employed for this purpose. U.S. Pat. No. 4,641,927 also to Prescott et al again teaches the use of GRIN rod lenses with a homogeneous lens following a transfer module of an endoscope to provide chromatic aberration correction, and additionally teaches that a GRIN element can be employed as an objective lens.
U.S. Pat. No. 4,723,843 to Zobel suggests that it is advantageous to use a plurality of relatively short GRIN lenses for a transfer module of an endoscope, rather than a single elongated GRIN rod lens, to reduce breakage. Images of the object are formed at the center of each of the elements of the transfer module.
U.S. Pat. No. 4,735,491 to Takahashi indicates that it is known to use the so-called "SELFOC" GRIN rod lenses for both the objective and the elements of the transfer module of an endoscope, but indicates that to do so leads to high chromatic aberration and additional image degradation. The specific improvement proposed by Takahashi is to employ a homogeneous distal objective lens, using the GRIN materials only for the lenses of the transfer module such that chromatic aberrations are reduced, and to make one of these axially movable together with one of the objective lenses for focus adjustment.
U.S. Pat. No. 4,755,029 to Okabe indicates that it is conventional to use a plano-ended GRIN rod lens as a distal objective lens for an endoscope, that is, at the tip of the endoscope. The specific improvement proposed by Okabe is to employ a distal GRIN objective lens having either a planar or a convex distal surface, and a convex proximal surface. Okabe provides an aperture stop outside the distal objective lens and suggests that a fiber optic bundle can be employed as a transfer module.
U.S. Pat. No. 4,783,154 to Takahashi states that it is conventional to use a GRIN rod-like lens in a transfer module of an endoscope. The invention proposed by Takahashi involves employment of homogeneous rod-like lens components in the transfer module, each associated with a GRIN lens. The GRIN lenses may be plano-concave, concave-convex or concave-concave.
U.S. Pat. No. 4,838,247 to Forkner teaches that it is generally conventional to employ GRIN lenses as elements of a transfer module of an imaging endoscope.
U.S. Pat. No. 4,895,433 to Takahashi teaches the use of a GRIN material for the prism of an endoscope, such that the field of view of the endoscope is centered about an axis at an angle to the optical axis of the probe, as well as use of GRIN materials for other optical elements of the objective. Takahashi's GRIN prism is neither "axial GRIN" or "radial GRIN" as these terms were defined above, as the variation in refractive index is not symmetric about the optical axis. Rather, the index of refraction varies linearly transversely across the prism, the gradient being at an angle to the optical axis. In at least one embodiment (see FIGS. 15A and 15B, column 7, lines 30-53), Takahashi suggests integral formation of the prism with a plano-concave element, such that both exhibit the same variation in index of refraction. Takahashi suggests that a fiber optic bundle should be employed as the transfer module.
Among other prior art documents not specifically directed to designs of endoscopes employing GRIN materials, U.S. Pat. No. 4,930,880 to Miyauchi teaches that in general an "axial GRIN" lens, in which the index of refraction varies axially rather than radially, and having a spherical optical surface, can be equivalent to an aspheric lens of a homogeneous, that is, non-GRIN material. Miyauchi does not discuss the application of this broad principle to design of an endoscope.
U.S. Pat. No. 4,859,040 to Kitagishi et al teaches several embodiments of telephoto and zoom lenses using elements formed of both positive and negative radial GRIN materials, including materials exhibiting both axial and radial GRIN characteristics. Kitagishi et al state at col. 11, lines 13-15 that the invention disclosed "may be applied to observation systems or illumination systems of microscope, telescope or others".
Finally, U.K. Patent No. 1,277,496 of Nippon Selfoc Kabushiki Kaisha discusses the manufacture of positive and negative radial GRIN materials of transparent polymeric resins. As noted above, GRIN materials may also be made of glasses having materials which alter the index of refraction diffused into the glass during manufacture.
However, despite the presence in the prior art of various suggestions for the use of GRIN optical elements for various optical instruments, and specifically for endoscopes, there has as yet been provided no imaging endoscope manufacturable sufficiently inexpensively as to be cost-effective for single-patient, disposable applications.